This invention relates generally to a device for controlling hydraulic working circuits and, more specifically, it relates to a control device that is applicable as a travel regulating valve for controlling a hydraulic power lift unit in a tractor. The device is of the type where a mechanically actuated slide of a preselector valve cooperates with a slide of a main selector valve that in response to the position of the preselector slide resumes a neutral position or an operative position. In the neutral position the main selector valve brings the supplied pressure fluid into a non-pressurized circulation whereas in the operative position the flow resistance in the non-pressurized circuit is increased and the pressure is applied to the load.
Hydraulic control devices of this type are known, for example from the German Pat. No. 1,928,896 and include besides the preselector valve slide and the main valve slide an auxiliary slide that is arranged in an axial boring of the main valve slide. In this known control device the auxiliary slide contributes to the pressure relief of a control chamber arranged in the main control slide. This prior art device is supposed to prevent by means of jumping movement of the auxiliary slider the occurrence of such a choking action in which an amount of pressure oil delivered by the pump is capable of leaking over the pressure relieving openings when pressure conditions have correspondingly increased so that the pump has to operate at an unduly increased pressure. This choking action or process, called "tight choking", may very quickly result in breakdown of the pump. However, in the prior art control devices the tight-choking is avoided during the pressure building phase that means during the displacement of the slider of the preselector valve from the neutral position to a lifting position, but there still remains the possibility that during the displacement of the preselector slider from working or lifting position to the neutral position pressure conditions may result that can induce the tight choking effect. Moreover, the known control device has the disadvantage that the tolerances and mutual arrangement of ports and control edges in the borings and on the slides of the valve, are critical. Above all, the prior art control device has the disadvantage that it is very expensive because in addition to the auxiliary slide there are necessary additional control shoulders, ports and control openings for diminishing the danger of tight choking.